According to the Wikipedia article on swans, swans are mostly monogamous but may sometimes separate, particularly after a "nesting failure". What "nesting failure" may mean is not entirely clear to me. Does it mean that they they have literally failed to build a physical nest? Or is it that they have failed to produce offspring? Or is it something else entirely?

Egg survival was defined as the proportion of eggs in successful nests that hatched (Johnson et al. 1992). Successful nests were those from which at least one egg hatched. Egg survival is referred to by some authors as hatching rate (Willey and Halla 1972). Eggs left in the nest after incubation ceased were considered abandoned.

and

We determined the cause of failure for 33 nests: 46% were flooded, 27% were abandoned, 15% were disturbed by humans, 6% were lost to predators, and 6% failed after a parent died.

Actually, nesting failure means that nesting trial fail before offspring could even leave the nest. In case of precocial species (where chicks leave nest immediately after hatching, like swans) it means that eggs was destroyed or parents abandoned nest, as @nico point out. In case of altricial birds (offspring stay in nest) nesting failure may also mean that all nestlings died before leaving nest.

About 81%
of nesting failures (n = 30) were due to predation (contents disappeared before the expected fledgingdate). Females abandoned seven nests, and starvation (presumably because
of desertion by parents) accounted for one nesting failure.